In an SAW device, if surface wave is reflected at the end surface of an element, this has influences on the characteristics of the device. For example, in an SAW filter, this causes worsening in the insertion loss of the filter or ripple in the band. In order to prevent the reflection at the end surfaces of the element, heretofore a method was generally adopted, by which sound absorber is located at the neighborhood of the end surfaces. FIG. 6 is a top view of such a device, in which reference numeral 1 is a piezoelectric substrate; 2 indicates interdigital electrodes; and 3 represents sound absorber.
On the other hand, in an SAW device having a piezoelectric thin film/semiconductor substrate) structure, apart from the method, by which the sound absorber is located, it is possible to prevent the reflection of the SAW also by a structure indicated in FIG. 7, in which the reference numerals used in common in FIG. 6 represent items identical or corresponding to those indicated in FIG. 6 and 4 is a semiconductor substrate; 5 is a rear electrode; and 6 indicates metal electrodes.
In the structure indicated in FIG. 7, the SAW passing under the metal electrodes 6 is attenuated by applying a bias voltage V.sub.B between the metal electrode 6 and the rear electrode 5 to convert the surface of the semiconductor substrate and as the result it is possible to prevent the reflection of the SAW at the end surfaces. The structure indicated in FIG. 7 has been already explained in detail in a Japanese patent application entitled "Surface Acoustic Wave Device" (Application No. 59-210566) filed by the same assignee.
However, the method has a drawback that in a small device it is difficult to arrange the sound absorber in a satisfactory manner. Further, it is difficult to keep the position, where the sound absorber is located, the shape and the amount thereof, etc. constant, which gives rise to another drawback that it may be a cause of fluctuations of products.
The structure indicated in FIG. 7 has an advantage that the drawback of the structure indicated in FIG. 6 can be overcome, because it can be fabricated by the photolithographic technique. However, for the structure indicated in FIG. 7, since a bias power source V.sub.B is necessary, in the case where the element is applied to a system, it has a drawback that the number of parts increases in the whole.